Alloys having an aluminum-diffused surface layer

ABSTRACT

IT IS KNOWN TO PROTECT THE SURFACE OF ALLOYS WITH A LAYER OF DIFFUSED ALUMINUM. IT HAS NOW BEEN FOUND THAT THE OXIDATIVE RESISTANCE OF CERTAIN ALLOYS, NAMELY HIGH TEMPERATURE STEELS, NICKEL BASED AND COBALT BASED ALLOYS, IS IMPROVED IF THE ALUMINUM LAYER IS DIFFUSED ONTO A SUBSTRATE OF NICKEL.

Aug.3, 1971 BUNGARDT ETAL 3,597,172

ALLOYS HAVING AN ALUMINUM-DIFFUSED SURFACE LAYER Filed Jan. 29, 1968 III Ivy/m1 United States Patent 3,597,172 ALLOYS HAVING AN ALUMINUM-DIFFUSED SURFACE LAYER Karl Bungardt, Krefeld, Gottfried Becker, Dusseldorf, and Giinter Lehnert, Krefeld, Germany, assignors to Deutsche Edelstahlwerke Aktiengesellschaft, Krefeld, Germany Filed Jan. 29, 1968, Ser. No. 701,317 Claims priority, application Germany, Jan. 31, 1967, D 52,148 Int. Cl. 1332b 15/00 U.S. Cl. 29194 4 Claims ABSTRACT OF THE DISCLOSURE It is known to protect the surface of alloys with a layer of diffused aluminum. It has now been found that the oxidative resistance of certain alloys, namely high temperature steels, nickel based and cobalt based alloys, is improved it the aluminum layer is diffused onto a substrate of nickel.

This invention relates to an improvement in the resistance to oxidation and corrosion of parts made of certain alloys, namely high temperature steels and nickel-based or cobalt-based alloys, having aluminum diffused into their surfaces.

The diffusion of aluminum into the surface of parts is a known method of improving the oxidation and corrosion resistance thereof. The conventional procedure consists in packing the objects into a powder mixture substantially consisting of metallic aluminum and aluminum oxide as an inert component. By raising the temperature to between 1000 and 1100 C. the metallic aluminum can be made to diffuse into the surfaces of the parts and to form a diffusion layer which protects the parts against oxidation and corrosion during their subsequent use.

Parts consisting for example of an alloy containing about 70% nickel, 20% chromium, remainder accompanying elements such for example as titanium, cobalt, silicon, manganese or iron, superficially oxidise when heated in air to about 1200 C., the oxide layer continuously forming and flaking off without a significant incu bation period. However parts made of the same alloy but provided with an aluminum layer diffused into their surfaces resist oxidative attack under the same conditions for 150 to 200 hours.

The invention relates to a further improvement of the known process of aluminum diffusion, in that high temperature steels, nickel-based alloys and cobalt-based alloys parts are provided with a nickel coating before they are submitted to the process of aluminum diffusion.

The accompanying drawing is a graphical illustration of the advantages obtainable by the process of the invention in comparison with untreated parts or with parts that have been submitted only to the aluminum diffusion treatment. The ordinate of the said graph represents the loss in weight per unit of surface area by heating in still air at 1200 C., and the abscissa represents time.

The comparative tests were performed using a nickel based alloy containing 20% chromium, 2.4% titanium and 1.4% aluminum. Aluminum was diffused into the surface by heating samples at 1100 C. for a period of 5 hours in a powder mixture containing 5% aluminum and 95% aluminum oxide. The sample that was treated according to the invention was first degreased and pickled and before being subjected to the aluminum diffusion treatment it was electrolytically provided with a a coat- "ice ing of nickel by a minute treatment in a hydrochloric acid nickel chloride bath at 35 C. using a current density of 25 a./dm.

It will be seen from the graph that the life of the 5 sample which had been provided with a nickel undercoating (shown in curve C) was substantially longer than that of the sample that had been provided with an aluminum diffusion layer in conventional manner (shown in curve B), which was itself substantially better than an 10 untreated sample, shown in curve A. This result was surprising in view of the fact that the nickel alloy of the parts submitted to the comparative tests as such already contained a major proportion of nickel, and it was not therefore to be expected that the resistance to oxidation 15 and scaling would be very substantially improved by the provision of an underlayer of nickel.

An improvement in the life of parts provided with an undercoating of nickel compared with parts that have been merely aluminised by diffusion was also found to occur with an alloy containing 43% cobalt, 20% chr0- mium, 20% nickel, 4% each of tungsten, molybdenum and niobium, 0.4% carbon, remainder iron, silicon and manganese.

The advantages which the above tests confirm can also be achieved by applying the nickel coating and the aluminised layer by methods other than those herein described.

It is preferred that the thickness of the nickel coating is from 3 to 20 1, particularly from 5 to 10 1., and the thickness of the aluminum diffusion layer is preferably from 20 to 100M, Particularly from to 70 The invention may be applied with advantage to the production of blading for turbines which is exposed at high temperatures to the oxidative and corrosive effect of combustion gases.

What is claimed is:

1. A part made of an alloy selected from the group consisting of a high temperature steel, a nickel based alloy and a cobalt based alloy, said part having a nickel coating; and said part having aluminum diffused into the surface thereof from exteriorly of said nickel coating, said part being thereby provided with improved oxidation and corrosion resistance.

2. The part of claim 1 wherein the part is a turbine blade.

3. The part of claim 1 wherein the nickel coating has a thickness of from 3 to 20,11. and the aluminum has a thickness of from 20 to 100,14.

4. The part of claim 1 wherein the nickel coating has a thickness of from 5 to 10 and the aluminum has a thickness of from 40 to 70 References Cited UNITED STATES PATENTS 2,611,163 9/1952 Schaefer et al 164-75 2,687,565 8/1954 Schaefer et al. 29196.2X 3,000,755 9/1961 Hanink et al 29196.2X 3,044,156 7/ 1962 Whitfield et al. 29l94- 3,129,069 4/1964 Hanink et a1. 29l94X 3,141,744 7/1964 Couch et al. 29194 3,450,512 6/1969 Maxwell 29l94X 3,477,831 11/1969 Talboom et a1 29l94X L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner U.S. Cl. X.R. 

